Disk-shaped and annular force sensors based on piezoelectrics and strain gauges are available commercially. For about 20 years, quartz load washers have become known for measuring dynamic single- and multi-component force processes. This design is characterized by the crystals being arranged between two rigid and solid disks, with the introduced forces transmitted for the most part through the piezo disks. This results in an overall height about two or three times the height of a standard washer. Examples are shown in DP 1 929 478 and CH 476 990 which corresponds to U.S. Pat. No. 3,614,488 and 3,582,691 respectively. As a result of modern amplifier technique, it is enough to detect only part of the effective forces, which is effected most simply by paralleling the force. This however is conditional upon the relation .alpha. where ##EQU1## not changing during the observation time. Values of .alpha. may range from 2 to 10,000 depending on application. A force sensor of this kind in a rectangular platform is shown in DE 34 40 670 C2. In such measuring plates, commercial piezo force measuring disks are glued. In these plates, very critical installation heights in relation to the measuring plate surface must be modified to within a few microns by adapter plates and thrust washers. One consequent of this serial array of disks (the commercial force measuring disk already has five of them) is a great uncertainty in the detection of the paralleled force transmission. This is because the contact surfaces of eight disks arranged in series have ten air gap layers, which have different ratios of elasticity depending on preloading. Owing to the ten different air gaps, the individual force measuring disks of such a measuring plate will always give different force measuring signals under the same loading, because the effects of the air gap layers are difficult to bring under control.
The purpose of this invention is to provide measuring disks whose measuring elements have not more than half as many air gaps, i.e. less than five. To achieve this, the disks are welded singly into the measuring elements under high preload. After this, the disk surfaces are ground over and possibly lapped together with the cover disks of the measuring elements, to obtain a perfect base or common planar surface. The measuring element cover disks can then be coated with an exactly determine vapor-deposition layer using a stencil. This enables the thin disk force sensor, according to the invention, to be fitted much easier, more accurately and reliable without additional adapter plates and thrust washers to be matched. Moreover, the overall height can be kept much lower, equivalent to that of standard washers, because no commercial force sensors are fitted in it but only components of the force sensors.
According to the invention, measuring elements are fitted in the sensor which may be sensitive in all three space coordinates X, Y, Z as well as to moments M. The defined force paralleling is obtained by vapor deposition onto the ground-over state. The reduction of the number of air gap layers and their minimization by welding under high mechanical preload allows using standardized disk elements and creates a new category of thin disk force sensors.
The invention is based primarily on piezoelectric measuring principles used with impedance transformer or charge amplifier. Quartz (S10.sub.2) is employed usually as piezo material, allowing measuring cycles lasting up to 15 minutes in conjunction with modern charge amplifiers, so that perfect static calibration is possible. With impedance transformers only brief phenomena can be detected, but these can be integrated in simple fashion into the sensors. Commercial impedance transformers for pressure and force sensors are described in the literature under the trade name "Piezotron transducers".
The use of piezo-ceramics yields stronger signals, but they are usable only for short-period measurements, while static calibration is possible though difficult. Nevertheless, they allow much smaller elements, which may offer advantages. However, since quartz plates can be stressed in both compression and shear depending on the crystal out, quartz has proved to be the ideal material for multicomponent dynamometry.
The invention may embody other known sensor techniques however. Thus the use of thin and thick film processes is conceivable, also capacitive principles, though only for single-component force measuring. Nevertheless, the piezoelectric principle is far superior owing to the high rigidity attainable.
The following remarks refer exclusively to piezoelectric arrangements therefore.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.